1. Field of the Invention
The present invention generally relates to high precision electron beam projection/deflection systems and, more particularly, to electron beam projection/deflection systems including at least one variable axis lens.
2. Description of the Prior Art
Electron beam projection/deflection systems are used in many types of devices such as cathode ray tubes used for display, as in televisions, oscilloscopes and computer displays. Electron beam projection/deflection systems are also used in electron beam lithography systems, particularly for exposure of resists in the fabrication of masks for use in fabrication of integrated circuits and also direct writing for patterning of resists directly placed on substrates and other structures. As integrated circuit density has increased and pattern feature size correspondingly reduced, exposure of resists using an electron beam exposure system, sometimes referred to as an e-beam tool, in which an electron beam can be deflected over a surface with high speed and accuracy under computer control, has become highly advantageous and the exposure methodology of choice.
Electron beam projection/deflection systems (hereinafter also referred to simply as deflection systems) are typically pivotal. That is, the beam trajectory is deflected around a pivot point on the electron-optical axis of the system at a finite distance from the target. There are two major consequences of this property of electron beam projection/deflection systems. First, for a substantial portion of the travel of electrons through the electron beam projection/deflection system, the electrons are separated from the electron-optical axis and are therefore subject to "off-axis electron-optical aberrations which degrades the performance of the electron beam projection/deflection system. Second, the direction from which the electron beam impinges on the target plane through the electron beam projection/deflection system (e.g. the plane of the surface of a substrate or specimen) varies with the impact position in the field which can be written by the deflected beam. Therefore, any axial displacement of the target surface, due, for example, to variation in thickness of a substrate or tilt of its plane relative to a plane normal to the electron-optical axis of the system, translates into an undesirable lateral displacement of the beam. Compensation of this lateral displacement generally requires complicated special measures to obtain electron beam exposure at the desired location.
However, the above-described deficiencies are largely eliminated in advanced projection/deflection systems employing the Variable Axis Immersion Lens (VAIL) concept. U.S. Pat. Nos. 4,544,846 to Langner et al and 4,859,856 to Groves et al, both assigned to the assignee of the present application are exemplary of the VAIL system and are hereby fully incorporated by reference herein.
Ideally, the VAIL system provides telecentric imaging as well as positioning of the electron beam within the exposure field of a lithography system. The term "telecentric" here means that at any location in the field the electrons in the beam appear to originate from an object on the optical axis infinitely remote from the target. This is accomplished by a collimator lens, which collimates the beam prior to deflection through complementary angles by a magnetic double-deflection device, and by altering the optical axis of the projection lens relative to the mechanical axis of the electron beam column to coincide with the location of the collimated beam. The latter is accomplished by superpositioning the magnetic field of an additional deflection device over the lens field. In practice, this arrangement seeks to maintain an angle of incidence on the target normal to the target plane.
However, it has been established that the VAIL system in its present implementation performs this function accurately only to a first-order approximation. Higher (mainly third) order effects lead to a landing angle variation and position distortion over the field of exposure large enough to be of concern for lithography of ever decreasing pattern feature dimensions for higher degrees of integration density of integrated circuits.
At higher component densities in integrated circuits, the tolerance for positional error in exposure by the electron beam becomes very much reduced. For pattern design rules having a particular minimum feature size, often referred to as a regime, the tolerance for errors of position and distortion of the electron beam is usually more than an order of magnitude smaller. These errors can originate from several sources and the total cumulative allowable error is commonly referred to as an error budget. For example, in a regime having a minimum feature size of a substantial fraction (e.g. about one-quarter) of a micron, the total error budget is on the order of a few tens of nanometers.
Keeping the shape and positioning of the electron beam deflection within this error budget is difficult. For example, in the VAIL system, the aforementioned third-order landing angle variation over a 10 mm deflection field amounts to a significant fraction of a degree. This can lead to positional errors of more than 100 nm, far more than is tolerable even for the 0.5 .mu.m regime. (Note that the angle variation of a pivotal (i.e. non-telecentric) projection/deflection system is more than an order of magnitude larger). Therefore, even with the superior performance of the Variable Axis Lens projection/deflection system the errors due to higher order variation from ideal telecentric performance are not acceptable for advanced semiconductor microfabrication.
Furthermore, the higher order angle and distortion aberrations of VAIL vary, by definition, not only non-linearly over the field, but also anisotropically (i.e. varying in both their radial and azimuthal components). Correction of these aberrations by design of the electron optics is not possible. Dynamic correction (i.e. beam position correction point by point) in the deflection field, based on a calibration procedure, is presently practiced. Dynamic correction, however, is excessively demanding as well as inadequate in its accuracy, particularly for the sub-half micron regime due to the need to obtain empirical correction data during calibration for a great number of points in order to approach the complexity of the aberrations.